<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <title>凹</title>
</head>
    <style>
        div{
            width: 300px;
            height: 300px;
            border: 1px solid #000;
            background: rgba(0,0,0,.8);
            border-radius: 50%;
            box-shadow: 0 0 10px 10px rgba(0,0,0,.1) inset;
        }
    </style>
<body>
    <div></div>
</body>
</html>